Injecting aqueous ammonia directly into the exhaust system upstream of an SCR catalytic converter (Selective Catalytic Reaction) is used to convert the NOx emissions in the exhaust gas into nitrogen and water vapor. The temperature of the exhaust gas should be above a minimum threshold temperature of at least 220° C. in order to ensure correct vaporization of the ammonia or urea.
In order to achieve the required degree of NOx conversion, injection of ammonia typically starts while the exhaust gas is still at a temperature below the required optimum threshold temperature, e.g. at 170° C. Moreover, it may be desirable to employ a mixer in the exhaust gas in order to ensure improve mixing of the injected ammonia with the exhaust gas. Injection at low temperatures can lead to poor vaporization of the injected aqueous ammonia and furthermore to deposition of ammonia in and on the injection nozzle and in the exhaust tract.
The ammonia deposits can have a negative effect on the quality of mixing and hence on the ability of the SCR catalytic converter to convert the NOx emissions. Finally, the deposition of ammonia can lead to a deviation between an expected or desired amount and the actual amount of ammonia injected, especially at low temperatures and/or when there are already deposits on the injection nozzle and/or the mixer. This may impair accuracy of an ammonia memory model and of the alpha ratio (moles of ammonia to moles of NOx) in the upstream part of the exhaust stream.
The description herein may improve the quality of ammonia injection in the after treatment system on a motor vehicle. According to the description, improved ammonia injection may be achieved by the features of the claims.
The inventor herein has recognized the above-mentioned disadvantages and has developed an engine system, comprising: an engine including an exhaust passage; an ammonia injector positioned in the exhaust passage; a differential NOx and ammonia sensing device sensing exhaust gases upstream and downstream of the ammonia injector; and a controller including instructions for adjusting engine operation in response to an output of the differential NOx and ammonia sensing device.
By determining an amount of ammonia deposited in an exhaust system via a differential sensor that is capable of sensing NOx and ammonia, it may be possible to regenerate or improve performance of emissions control devices including sensors and actuators via an ammonia purge cycle. In one example, ammonia deposited on an ammonia injector can be removed via increasing temperature at the tip of the ammonia injector. In other examples, exhaust gas temperatures can be increased so as to promote release of ammonia deposited in the exhaust system.
The present description may provide several advantages. Specifically, the approach may reduce engine emissions via improving mixing of ammonia in exhaust gases to facilitate NOx reduction. Further, the approach may be useful for determining degradation of an ammonia injection system so that an operator can be prompted to take measures to correct the degradation and thereby reduce engine emissions. Further still, the approach may help to reduce emissions of ammonia from a vehicle.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
The drawings included herein serve merely for the explanation of the description, and do not restrict the description. The drawings and the individual parts are not necessarily drawn to scale. The reference symbols that are common to more than one figure are used to denote identical or similar parts between different figures.